FWIW, I struggled with exactly the same issues with Wenthish, which likewise is a Germanic language that had influence from Old Irish. It's ended up not looking that Irish at all - other than some loanwords and maybe some spelling conventions - but I've gone through several phases of Celticising it, including toying with mutations. [and this thread make me want mutations again, damnit!]

In the end, what (mostly) resolved it for me was that I decided to have mutations, and couldn't work out how. The way the obvious culprits had developed, mutation just couldn't have had a sufficiently significant burden for it to have become systematic.

[a particular problem: Celtic had nice -s endings on things. But Germanic has weak little -z instead, and final -z drops entirely outside north germanic (and monosyllabics in irmionic). So, for instance, the masculine and feminine articles are likely to trigger the same mutation. And the feminine singular and plural must do as well. And so on.]

That's your problem, you should have made it North Germanic like me!

Which articles have -z? In North Germanic you get three articles, two of which end in nasal consonants.

I partially solved this problem by adopting Irish "na" for the feminine while keeping masculine en (<hinn). The neuter et causes t-prothesis (ett hús > et tú(s)) "DEF.N house" although it seems that the neuter is slowly losing out.

(Nb I'm not quite sure whether I want to leniting final s and whether it'd also apply to -z. Likely it would unless continental influence is strong enough to prevent it. )

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Current Projects:Mannish — A North Germanic language spoken on the Calf of ManPelsodian — A Romance language spoken around Lake BalatonJezik Panoski — A Slavic language spoken in the same area

Seen this thread a hundred times; finally thought of something to say!

Most of my conlanging arises from conworlding. I'm working on some aspect of the culture, and I need a word, or a phrase, and it all takes off from there ... So maybe laying languages aside and focusing on other aspects of the world would actually help you work on those languages.